The biological functioning of some proteins is critically dependent on their state of self association or polymerization. Although a number of such systems is known and much information on the effect of external conditions on the association is at hand, mechanisms are still largely unknown. Knowledge of the stoichiometry and the thermodynamic properties of the associations are quite useful in forming hypotheses on the mechanism; with simple systems, this can usually be obtained in a straightforward manner. With some systems such as the protein from tabacco mosaic virus, however, the association or polymerization is quite complicated. Knowledge of the stoichiometry and bond energy, especially of the structural intermediates, is most difficult to obtain. Yet, this is necessary to understand the overall process. Because of its importance, our objective is to explore ways of obtaining such information. Equilibrium sedimentation measurements show much promise in this respect, at least for the initial stages of more complicated associations. Concerning the refinement of the technique for such applications some preliminary data and work by the principal investigator and colleagues exist. One of the immediate goals is to develop this approach for the protein of tobacco mosaic virus, utilizing supplemental techniques when needed. Not only is the information useful in itself, it is likely that the approach can be used on other systems.